1. Field of the Invention
The present invention relates to a control device for an in-cylinder injection internal combustion engine that makes fuel have high pressure through a high pressure pump to supply fuel to an injector and then injects fuel directly into a cylinder from the injector.
2. Description of Related Art
In an in-cylinder injection engine that injects fuel directly into a cylinder, a time from injection to combustion is short as compared to an intake port-injection engine that injects fuel into an intake port, so that enough time for atomizing the injected fuel cannot be gained. Accordingly, the injected fuel needs to be atomized by increasing injection pressure. For this reason, as described in JP-A-10-331734 (for example, p. 3), in the in-cylinder injection engine, fuel, which has been pumped-up by a low pressure pump from a fuel tank, is made to have high pressure through a high pressure pump that is driven by a cam shaft of the engine so as to be pressure-fed to an injector.
With respect to behavior of fuel pressure in a high pressure fuel pipe from this high pressure pump to the injector after the engine stops, fuel temperature rises in accordance with a rise of engine temperature due to residual heat of the engine and thereby the fuel pressure increases for a short while immediately after the engine stops. After that, in accordance with gradual decrease of the engine temperature due to heat release so that the fuel temperature is gradually reduced, the fuel pressure is gradually reduced. Moreover, in the in-cylinder injection engine, the fuel pressure is controlled to be a high pressure (e.g., about 8 MPa), even when the engine is in idle operation immediately before the engine stops. Therefore, coupled with the fuel pressure behavior after the engine stops as above, a period during which the fuel pressure is kept high after the engine stops becomes long. In addition, as the fuel pressure while the engine is at a stop is higher, an amount (oil tight amount) of fuel which leaks out of the injector becomes larger. Because of these circumstances, in the in-cylinder injection engine, the amount of fuel leakage while the engine is at a stop tends to be large as compared to the intake port-injection engine whose injected fuel pressure is low, and the leaked fuel accumulates in the cylinder so as to be discharged remaining unburned at the time of the following start-up of the engine, so that exhaust emission at the time of the start-up of the engine deteriorates.
As described in JP-A-2007-46482 (for example, pp. 2-4) corresponding to US2007/0028897A1, in order to solve this problem, it is proposed to decrease fuel pressure in the high pressure fuel pipe immediately before the stop of the internal combustion engine by determining whether the internal combustion engine is about to stop based on at least one of an idle operation command, an operative position of a shift lever, and a vehicle speed and then by setting a target fuel pressure on a lower-pressure side than usual if it is determined that the engine is about to stop.
However, according to the technology of JP-A-2007-46482, when it is determined that the internal combustion engine is about to stop, the target fuel pressure is reduced in a stepwise fashion. Therefore, a difference (hereinafter referred to as “fuel pressure difference”) between actual fuel pressure (fuel pressure detected by a fuel pressure sensor) and target fuel pressure increases in a stepwise fashion immediately after the reduction of target fuel pressure. Therefore, in the fuel pressure control system that feedback-controls a discharge amount of the high pressure pump based on the fuel pressure difference to feedback-control fuel pressure, the fuel pressure difference, which is increased in a stepwise fashion immediately after the reduction of target fuel pressure, is accumulated and the integral term of the feedback control negatively becomes excessively large. As a result, undershoot is generated so that the actual fuel pressure falls to well below the reduced target fuel pressure. Accordingly, the actual fuel pressure may become too low so as to worsen combustion, and exhaust emission may deteriorate.